The invention relates to a composition that includes a silicon-containing substrate and a zircon coating.
Silicon-containing substrates are proposed for structures used in high temperature applications, such as in heat exchangers and advanced internal combustion engines. For example, silicon-based composite ceramics have been proposed as materials for applications in combustors for supersonic commercial airplanes. In many of these applications, the silicon-containing substrates are subjected to highly corrosive environments such as oxidative or reducing atmospheres and environments containing salts, water vapor or hydrogen. Silicon-containing substrates exhibit poor oxidation resistance and they may recede and lose mass under water-containing environments because of the formation of volatile species, such as silicon hydroxide [Si(OH)4]. Hence, it is necessary to apply external environmental/thermal barrier coatings to these materials to provide protection from environmental attack at elevated temperatures.
Although an external environmental/thermal barrier coating is capable of preventing silicon-containing substrate materials from being in direct contact with the environment, typically the external coating is an oxide that has a high coefficient of thermal expansion (CTE). The difference between the coefficient of expansion (CTE) of the silicon-containing substrate and the coeffcient of thermal expansion (CTE) of the external environmental/thermal barrier coating can result in high stress and lead to coating failure. Further mullite, which is used as an intermediate bond coating between a silicon-containing substrate and an oxide external environmental/thermal coating, tends to crack because of the coefficient of thermal expansion (CTE) mismatch with a silicon carbide/silicon (SiC/Si) substrate. These cracks are detrimental to the function of the ceramic coating because they serve as fast transport paths for oxidizing and corrosive species to cause severe oxidation and corrosion at interface with the substrate. Additionally, cracks in the coating concentrate stresses. The cracks apply shear and tensile forces on the substrate to cause substrate fractures.
Thus, there is a need to provide a coating to a silicon-containing substrate that reduces stress and cracks in a ceramic composite.
The present invention provides a coating that reduces thermal expansion mismatch with a silicon-containing substrate and that can be applied as an intermediate layer or as an external coating in applications where water vapor is absent. The invention is an article that comprises a silicon-containing substrate and a zircon coating.
In another aspect, the invention relates to a method of forming an article, comprising forming a silicon-containing substrate and applying a zircon coating